PTEN is a tumor suppressor that is deregulated in a large number of human cancers to activate the oncogenic phosphoinositide-3 kinase (PI3K) pathway. Inactivation of PTEN leads to multiple phenotypes including enhanced cellular proliferation, increased glucose metabolism, migration, and survival. We have recently determined that PREX2 is an inhibitor of PTEN phosphatase activity that is regulated by insulin and involved in the regulation of glucose metabolism. PREX2 is over expressed in human cancers with wild type PTEN. Moreover, over expression of PREX2 is seen in cancer in the presence of mutations of the gene PIK3CA encoding the catalytic subunit of PI3K. We have shown that mutant PIK3CA and PREX2 can stimulate cell growth in vitro, and that reduced expression of PREX2 inhibits tumor cell growth in a setting of wild type PTEN. PREX2 encodes an enzyme that catalyzes the loading of GTP onto the GTPase RAC1 and is a mediator of cell migration, and we have recently demonstrated that PTEN is able to inhibit PREX2 activation of RAC1 in a PIP3-independent fashion. We propose that the mutual regulation of PTEN and PREX2 is likely to be regulated by phosphorylation based on our preliminary data. We plan to determine how this signal functions normally and is hijacked in cancer to drive proliferation and migration. This application will use a combination of biochemistry, cell biology, and mouse genetics to address the following goals: 1) to understand the mechanism of PREX2-mediated inhibition of PTEN and determine its effect on cells, 2) to understand the relationship between PTEN and PREX2 with regard to glucose metabolism, 3) to understand the mechanism through which PTEN inhibits PREX2 RAC-GEF activity and its implications for both normal cell migration and tumor cell invasion and 4) to determine the phenotypic consequences of cancer-derived PREX2 mutations and examine their basis of oncogenic action with regard to glucose metabolism, and the mutual inhibition of PTEN and PREX2.